In semiconductor lasers and related optical components the carrier injection that is involved typically produces simultaneous changes in both the optical gain and the refractive index of the material. The coupling of the two effects is unfortunate for a variety of applications. In amplitude modulation the index changes cause unwanted frequency chirp. In cw lasers the gain fluctuations that are induced to compensate the noise fluctuations induce index fluctuations which broaden the linewidth. By contrast, in frequency modulators it is the gain change that is unwanted. In more complex situations which involve delayed feedback, or the interaction between master and slave lasers, the coupling of the two effects causes noise and difficulties in synchronisation. Hence a means of overcoming the coupling would be very useful.
The total effect of carriers may be alternatively described as a complex change in permittivity, where gain contributes the imaginary component. The ratio of the real to the imaginary components is equal to the negative of the line broadening factor .alpha., which has been variously reported to lie between 2 and 8. A means of rotating the permittivity vector in the complex plane, to make it purely real or imaginary, is therefore desirable.
The permittivity that controls the operation in most optoelectric devices is not the bulk permittivity in the active layer but the effective permittivity of the optical waveguide involved. Changes in the latter are obtained from changes in the former, whether they are real or complex, by multiplication by a confinement factor .GAMMA., which depends on the waveguide configuration. When .GAMMA. is real .alpha. is unaltered. However, in certain circumstances .GAMMA. is complex, in which case .alpha. is changed.